The present invention generally relates to an optical character reader (OCR) apparatus. More particularly, the invention is concerned with a shading correcting method advantageously suited for use in correcting non-uniformity in the sensitivity of an image reader head (also referred to as image sensor) of the OCR apparatus.
In the OCR apparatus, reading of such a document having a uniform density may nevertheless result in non-uniformity in the output level of the image sensor signal generated through photoelectrical conversion of the document information. Particularly, there is observed a phenomenon in which the output level of the sensor signal produced at both sides of the document tends to become lower than the sensor output produced at the center portion of the document, whereby non-uniformity in density appears in the image information of the document as a whole. This phenomenon is known as the shading phenomenon. As the causes for the shading phenomenon, the following may typically be mentioned.
(1) Non-uniformity and variation in illuminance of a document illuminating lamp:
Typically, a fluorescent lamp is employed as a document illuminating lamp. In this case, because of a finite length of the lamp as well as inherently lower luminance of light emitted at both ends of the lamp than at the center portion due to light emitting mechanism of this sort of lamp, the illuminance is decreased at both ends of the lamp. Additionally, blackening occurs at both ends of the fluorescent lamp in the course of the use to provide non-uniformity and variation in the distribution of illuminance. Additionally, the manner of mounting the fluorescent lamp may provide a cause for such undesirable phenomenon.
(2) Attenuation of light intensity by the lens of optical system:
The quantity of light transmitted through a lens of an optical system is decreased at a peripheral portion according to a cosine biquadrate law. By way of example, at a half-circle angle of 20.degree., the quantity of light at the peripheral portion of the lens is decreased to 78% of the quantity of light at the center portion thereof.
(3) Non-uniformity in the sensitivity among photoelectric conversion elements:
The solid-state imaging elements such as those of a charge-coupled device (CCD) and photo-electric conversion element of a diode array are likely to exhibit non-uniform distribution of sensitivity due to tolerances involved in manufacture and fabrication.
Correction of the shading phenomenon in the OCR has heretofore been carried out by reading at first a white reflecting plate providing "white" reference before the reading of a document and storing the derived data as the reference data to be utilized later for correcting the shading phenomenon of the image data resulting from the document reading operation.
In the prior art document transport type OCR apparatus, the image reader head or image sensor is designed to scan constantly a same field of view through which the document is transported. In the case of the document transport type OCR apparatus, it is required for the correction of the shading phenomenon, i.e. correction of the non-uniformity in the output of the image sensor, to read at first the white reference by other means because of the absence of an operation for causing the image sensor to read first the white reference and then shifting the field of view to the document as with the case of a document fixed type (i.e. sensor moving type) OCR apparatus. In connection with the document handled by the OCR apparatus, it is noted that a sheet or paper of the document is usually printed with a format designating the positions of characters to be printed or scribed. Under the circumstances, the cut-out of the character images to be read out is performed with reference to the periphery of the document. To this end, the OCR apparatus is provided with a plate printed in black or a planar mirror (equivalent to the black plate because of the absence of illuminating light rays in the direction normal to the plane of the mirror) as a background for the field of view of the image reader head (image sensor) so that the periphery of a document can be detected when the document (having a white peripheral portion) enters the field of view.
For effectuating correction of the shading phenomenon for the output of the image reader head of the document transport type OCR apparatus, a white reflection plate providing the white reference is first moved into the field of view of the image reader head for thereby deriving a reference shading waveform (i.e. waveform serving as a reference for correcting the shading of image information). Thereafter, the white reflection plate is removed from the field of view of the reader head, whereupon the entry of a document to be processed is awaited for with the background being set in black.
The insertion and removal of the white reflector plate has heretofore been realized mechanically with the aid of a mechanism of a complicated structure including mechanical elements, a solenoid-type actuator, a power driver circuit and others.
A typical one of the prior art OCR Apparatus described above is disclosed in Japanese Patent Laid-Open No. 109975/1983 (JP-A-53-109975). Further, as the shading correction technique in the OCR apparatus, there may be mentioned those disclosed in JP-A-61-246890, JP-A-61-224781, JP-A-61-121181,JP-A-60-98765, JP-A-58-121864 and JP-A-59-193669.